Mre11-Rad50-NBS1 (MRN) is a highly conserved protein complex in eukaryotes, which is involved in DNA double strand break (DSB) repair, homologous recombination, DNA damage signaling and cell cycle regulation, and telomere length maintenance. The core of the complex, Mre11 has been widely studied in yeast and humans. Mre11 null mutants are embryo-lethal in mammals. Arabidopsis Mre11 mutants are viable, however, are associated with severe developmental abnormalities and male sterility.
In this study, I describe the structural and functional characterization of the two Mre11 genes, Mre11A and Mre11B and the NBS1 gene in maize. To my knowledge, this is the first study that demonstrates that the Mre11 gene is duplicated. An extensive search of fully or partially sequenced genomes of various organisms suggests that the duplication of the Mre11 gene is a unique event in three grass species; Oryza sativa , Sorghum bicolor and Zea mays . It has been recently reported that the A, B and D genomes of Triticum each bears only one copy of the Mre11 gene.
I also show that Mre11A is alternatively spliced and the alternative splicing event may be developmentally regulated. Mre11B expression is also developmentally regulated. In collaboration with Anne Sylvester (University of Wyoming), we showed that Mre11B protein is localized to the nucleus and it is undetectable in non-dividing cells, which suggests that Mre11B is strictly expressed in actively dividing cells.
I also describe the discovery of a plant homolog for the third, signaling component of the complex, the NBS1 protein in maize. ZmNBS1 includes all five signature domains only found together in NBS1 proteins of animals. I cloned the maize NBS1 gene and, by yeast two-hybrid assay, show it interacts with Mre11A but, interestingly, not with Mre11B. I also looked at the interaction between Mre11A, Mre11B and Rad50. In a yeast twohybrid system, Mre11B interacts with Mre11A but not with Rad50.
We have a ZmNbs1 null allele that was identified by TILLING and also two Mre11B TUSC alleles that were provided by Pioneer Hi-Bred International. Homozygous maize Nbs1 and Mre11B mutants are viable, fertile and do not show any obvious developmental phenotype.
